1. Field of the Invention
The present invention relates to an optical receiver that receives, with interferometers categorized as the space optical system, modulated light that has been modulated by a modulation method such as a differential quadrature phase shift keying system.
2. Description of the Related Art
In recent years, in order to meet the demand for increase in capacity and distance in an optical transmission system, a phase modulation system has been put into practice. For example, differential phase shift keying (hereinafter referred to as DPSK), differential quadrature phase shift keying (hereinafter referred to as DQPSK), etc. are used for the optical transmission system. Further, development has been made of a technology that uses interferometers categorized as the space optical system in a demodulator for demodulating an optical signal that has been modulated by one of those methods.
Japanese Patent Application Laid-open Nos. 2007-151026, 2009-300539, and 2009-300540 each disclose an optical receiver for demodulation of DQPSK modulated light, which uses two Michelson interferometers. The two Michelson interferometers share a prism for reflecting light. The first interferometer uses the inner side of the prism to reflect the light back, and the second interferometer uses the outer side of the prism to reflect the light back.
When the space optical system is used, there is a problem of difficulty in downsizing. For example, in technologies disclosed in Japanese Patent Application Laid-open Nos. 2007-151026, 2009-300539, and 2009-300540, not only the shared prism is upsized, but a member for optical path length adjustment needs to be specially provided in order to control a so-called skew. Thus, the downsizing is hindered. In this case, the member for optical path length adjustment is a member for adjusting optical path lengths of four light beams to be input to light receiving elements so that the four light beams have equal optical path lengths from the first element that splits the input light toward the two interferometers to the light receiving elements. Further, in the optical receivers disclosed in Japanese Patent Application Laid-open Nos. 2007-151026, 2009-300539, and 2009-300540, a problem of difficulty in characteristic control also arises. The optical receiver uses two interferometers having an asymmetric shape, that is, the interferometer on the inner side of the prism and the interferometer on the outer side of the prism. Because of this asymmetric property, even when an attempt is made to control the optical phase with use of the thermo-optic effect of a phase adjusting member, the optical path lengths of light beams traveling through the two interferometers become unequal to each other due to heat unevenness. Further, the degree of influence of substrate strain due to the heat differs between the two interferometers. Therefore, in the optical receivers disclosed in Japanese Patent Application Laid-open Nos. 2007-151026, 2009-300539, and 2009-300540, it is difficult to prevent cross-talks, signal loss, and fluctuations in polarization characteristics.